The present invention relates to planar diaphragm loudspeaker systems. More particularly, the invention relates to a coaxial type planar diaphragm loudspeaker system in which polygonal diaphragms are coaxially arranged.
It is known that a planar diaphragm has a relatively flat acoustic pressure characteristic throughout its frequency range thus providing high fidelity sound reproduction. Accordingly, loudspeaker systems employing planar diaphragms have been proposed in the art. One of the advantages of such a planar diaphragm loudspeaker system is that its acoustic image has very little shift. If a speaker system employs coaxial planar diaphragms, then the amount of shift of the acoustic image is further reduced.
In the construction of a coaxial planar diaphragm type loudspeaker system, a number of problems are involved with the configuration and assembly of various components such as the voice coil bobbin, voice coil, and magnetic circuit members. For example, for polygonal diaphragms, it is desired that the voice coil bobbin have the same configuration as the polygonal diaphragm in order to properly apply driving forces to the diaphragm. More specifically, for three planar diaphragms each having a circuit configuration with the three diaphragms coaxially arranged to form a 3-way loudspeaker system, it is necessary to provide driver units for the mid-range and treble-range inside a driver unit for the bass-range. Thus, the driver unit for the bass-range must be large in diameter to receive the remaining two driver units. This lowers the manufacturing efficiency of such a speaker system. One solution for this problem is to connect a number of small-diameter voice coils in parallel. However, that arrangement is disadvantageous in that the fabrication thereof is extremely complicated and that the back pressure in the speaker system is weakened due to a gap formed in the back surface of the diaphragms.
Another problem in constructing coaxial type loudspeaker systems is that the driving force cannot be uniformly applied to the entire surface of the planar diaphragm since the voice coil bobbin which transmits the driving force to the diaphragm cannot be attached to the diaphragm throughout the entire surface thereof. Accordingly, the driving force is concentrated in particular portions of the diaphragm. Furthermore, the voice coil bobbin must be provided in the vicinity of the outer circumferential edge of the diaphragm because of structual limitations. As a result, the mode of vibration is different between the region near the outer periphery of the diaphragm and the region near the inner periphery of the diaphragm and thus split vibration is liable to occur.
Still another problem in constructing coaxial type loudspeaker systems is that it is difficult to coaxially arrange the magnetic circuits if more than two external magnetic circuits are used in a single loudspeaker system. Normally, an external magnetic circuit is employed for the bass-range diaphragm and an internal magnetic circuit is employed for the treble-range diaphragm. It is for this reason that the external magnetic circuit is relatively large and hence it is difficult to arrange a second external magnetic circuit inside the first external magnetic circuit.